106 THE VERTEBRATE SKELETON. 



by the haemal arches ; those of the dorsal and anal fins and 

 of the dorsal part of the caudal fin generally by interspinous 

 bones, which in adult Teleosteaiis alternate with the neural 

 and haemal spines. In Dipnoi these interspinous bones arti- 

 culate with the neural and haemal spines. In many Siluroids 

 the anterior rays of the dorsal and pectoral fins are developed 

 into large spines which often articulate with the endoskeleton, or 

 are sometimes fused with the dermal armour plates. Similar 

 spines may occur in Ganoids in front of both the dorsal and 

 anal fins. Polypterus has a small spine or fulcrum in front of 

 each segment of the dorsal fin. Such spines are often found 

 fossilised, and are known as iclithyodorulites. 



Similar spines are found in many Elasmobranchs, but they 

 are simply inserted in the fiesh, not articulated to the endo- 

 skeleton. They also differ from the spines of Teleosteans 

 and Ganoids in the fact that they are covered with enamel, 

 and often have their edges serrated like teeth. In the extinct 

 Acanthodii they generally occur in front of all the fins, paired 

 and unpaired. 



In Trygon, the Sting-ray, the tail bears a serrated spine 

 which is used for purposes of offence and defence. Many 

 ichthyodorulites may have been spines of this nature fixed to 

 the tail, rather than spines situated in front of the fins. The 

 spines, which are always found in front of the dorsal fin in 

 Holocephali, agree with those of Elasmobranchs in containing 

 enamel, and with those of Teleosteans in being articulated to 

 the endoskeleton. 



TEETH. 



The teeth of fish 1 are subject to a very large amount of 

 variation, perhaps to more variation than are those of any 

 other class of animals. Sometimes, as in adult Sturgeons, 

 they are entirely absent, sometimes they are found on all the 



i See W. G. Eidewood, Nat. Sci. vol. vin. 1896, p. 380. Full references 

 are there given to the literature of the subject. 



